CHICAGO — Several weeks ago, President Obama was at an event in Chicago when he approached Willie Wilson, a businessman who had challenged Rahm Emanuel in the mayor’s race, with a gentle plea, Mr. Wilson said.

“He came up and said, ‘You know, you’ve got to vote for my boy Rahm, you’ve got to support my boy Rahm,’ ” Mr. Wilson said on Thursday, surrounded by a gaggle of supporters at a Baptist church on the South Side. “But I’ve got to say this today: Mr. President, I’ve got to support my boy Chuy.”

And with that pronouncement, and a whoop of cheers from the crowd, Mr. Wilson endorsed Jesús G. Garcia, known as Chuy, as the mayor’s race heads to a runoff April 7.

Mr. Emanuel won 46 percent of the vote in February; Mr. Garcia, a county commissioner and former alderman, won 34 percent. Mr. Wilson had 10 percent, nearly 50,000 votes.

Mr. Wilson’s endorsement, quietly sought by Mr. Emanuel and Mr. Garcia, was not terribly surprising, given that he had vowed that he would not vote for Mr. Emanuel.

But it could provide Mr. Garcia with a needed boost in the eyes of African-American voters, who have wavered in their support for Mr. Emanuel and may turn out to be a crucial constituency in April.

Among their complaints: Under Mr. Emanuel, nearly 50 underperforming or underutilized public schools have been closed, and a network of red-light cameras that many voters call draconian has been installed throughout the city.

Both candidates have focused their campaigning on the South Side, which is largely populated by black voters.

“I think Rahm Emanuel was insensitive to the needs of the people,” said Otis Davis Jr., a supporter of Mr. Garcia’s. “He felt he was invincible. Now, I think the mayor knows that he’s not.”

On Thursday, Mr. Garcia and Mr. Wilson walked together jubilantly into a room at the church where reporters had assembled, as “Ain’t No Stoppin’ Us Now” blared from loudspeakers.

Mr. Garcia criticized Mr. Emanuel for allowing the city to enter “financial free fall,” though he has spoken only in generalities when asked how he would solve Chicago’s financial woes. He has not explained how he would balance the budget or fulfill a campaign promise of hiring 1,000 new police officers.

“Remember that it was Chicago that invented the skyscraper,” Mr. Garcia said, adding that in the next four years, “our greatness in the 21st century will be ushered in.”

Mr. Wilson said he had spent weeks circling the city and talking to voters, particularly African-Americans on the South Side, to see which candidate they wanted him to endorse. “Some of the main concerns of the citizens were the same concerns that I have,” he said, adding that Mr. Garcia could unite people across races and ethnicities. “This is not what you call a black and brown issue here. This is about black, brown, white, Asian, all citizens of Chicago. And so we must push away some of the things that we’ve talked about in the past.”

In an interview, Mr. Wilson said he had texted Mr. Emanuel on Wednesday to tell him that he would endorse Mr. Garcia.

“I said I hoped we could always have a relationship in the future,” he said.

Did Mr. Emanuel write back?

“No, he did not respond,” Mr. Wilson said.

A version of this article appears in print on , on Page A12 of the New York edition with the headline: Key Chicago Figure, Despite a Plea by Obama, Gives a Lift to Emanuel’s Rival. Order Reprints | Today’s Paper | Subscribe